FIG. 12 shows a configuration of a conventionally-typical active matrix EL display device 101. The numeral 102 denotes a unit pixel contained in the active matrix EL display device 101. Though unit pixels 102 are arrayed in a matrix in an actual device, only one unit pixel is shown here for the sake of clarity. The unit pixel 102 includes an EL element 103, a driving transistor 104 connected to one end of the EL element 103, a switching transistor 105 connected to a gate of the driving transistor 104, and a capacitor 106. To a gate of the switching transistor 105, a scanning signal is supplied from a scanning-driving circuit 108 through a scanning line 107. To the gate of the driving transistor 104, an image signal is supplied from a signal-driving circuit 110 via the switching transistor 105 and a signal line 109. To the EL element 103, a current is supplied from a current supply circuit 112 via the driving transistor 104 and a current supply line 111.
A light emission operation of this EL display device 101 will be described below. First, when both the scanning line 107 and the signal line 109 are turned on, electric charge is stored in the capacitor 106 through the switching transistor 105. Subsequently, since this capacitor 106 continues to apply voltage to the gate of the driving transistor 104, the current continues to flow into the EL element 103 from the current supply circuit 112 via the current supply line 111 even when the switching transistor 105 is turned off, and thus light emission and driving are carried out based on the electric current corresponding to the current image signal until an image signal is rewritten in the next field.
In a case of displaying gradation by means of the conventional active matrix EL display device, it is possible to apply to the gate of the driving transistor 104 a voltage corresponding to the gradation so as to vary the ON current analogically. In this case, the variation in the ON current of the driving transistor 104 affects the display. The ON current of the transistor is extremely uniform for a transistor composed of single crystal. However, in a transistor formed with a low-temperature polysilicon that can be formed on an inexpensive glass substrate, the threshold value has a variation in a range of ±0.2 V to 0.5 V. As a result, the ON current flowing in the driving transistor 104 varies corresponding thereto, resulting in unevenness in the display. Variation in the ON current may be caused not only by the variation in the threshold voltage but also by variation in the mobility in TFT, variation in thickness of a gate insulating film, or the like. Therefore, in the above-described method of displaying the gradation analogically, their properties must be controlled strictly. However, this is difficult with the low-temperature polysilicon TFT in present use.
An area gradation display method is suggested for solving this problem. This method includes forming, within a unit pixel configuring an active matrix EL display device, a plurality of EL elements and a plurality of thin film transistors for supplying current to the respective EL elements, and controlling by the thin film transistors the number of EL elements to emit light in accordance with the gradation. According to this configuration, the variation in the thin film transistor properties will not appear as a variation in the brightness of the EL elements, providing an accurate gradation in display.
However, when the area gradation display method is used for displaying by means of an active matrix EL display device, a fixed pattern is generated on the display image, and thus the image quality deteriorates.